194 



GLEICHENIACEAE 



[CH. 



by mis-interpreting the arrested leaf-apex in GleicJienia as an adventitious 

 bud, A correct understanding of the leaf-architecture is obtained by study 

 of the ontogeny. Observations have been made on young plants of GleicJienia 

 {^Dicranopteris) linearis, in which the juvenile leaf may at first be con- 

 structed on a dichotomous system, as is usual in Ferns, but showing even 

 in the cotyledon advanced sympodial development (Fig. 473, a, d, cot.). 



Fig- 473- Juvenile leaves of Ghichenia {Dicranopteris) linearis, a, b, show ordinary pinnate types, 

 with sympodial development, c, d, e, show early stages of arrest of the apex, such as lead to the 

 characteristic habit of the genus : / is a cotyledon with its apex arrested after two pinnae have been 

 produced, (a, r, rfx 5 ; ^ x 4; e x 10 ; / x 30.) 



The later leaves are wholly so constructed, as appears from their apical 

 regions, the leaflets often showing distally a single forking of the veins : but 

 passing downwards there are clear steps to a scorpioid sympodium in their 

 venation {b). The leaf-construction thus initiated may be continued in- 

 definitely, though in some simple Gleichenias and in Platysoina the elabo- 

 ration does not proceed further than a primary pinnation. But the young 

 plants of G. linearis and of some others illustrate that the "false dicho- 



